The present invention relates to a method and apparatus for detecting pressure surges in a turbo-compressor.
Many methods are known for detecting a compressor surge. The most extensively used method involves monitoring the suction flow (intake volume) of the compressor. Whenever the suction flow falls below a prescribed minimum limit, it is assumed that normal throughflow has broken down and a surge is about to occur.
In this conventional method, the intake flow (volume) is measured by means of either an orifice or a nozzle positioned in the intake duct of the compressor. It is a drawback of this conventional method that the intake throttling device (the orifice or nozzle) causes a permanent pressure loss thereby increasing the total power consumption. Another drawback is that this method is not suitable for fully accurate operation. If an extremely fine or sensitive adjustment is made, the conventional method, under certain circumstances, may indicate surges although no surges have occurred; in the case of too coarse an adjustment, compressor surges might not be detected at all, under certain circumstances.
It has to be considered, when adjusting the system, that the flow at which surge begins varies with the load of the compressor. At low load, surge will start at low flow rates. If the load is increased, surge will start at higher flow rates.
Furthermore, another method is known which monitors the velocity at the compressor intake of the gas to be compressed. In this case, the gas velocity (which is proportional to the square of the flow rate) may be detected simply by comparing the static pressures in two positions of different flow cross-section, already present at the intake duct.
Advantageous with such a method is that the detecting orifice does not cause additional resistance to flow. However, a drawback is that the detecting system for measuring the gas velocity always provides a positive signal even if the flow direction has reversed under the action of a surge. In practice, a differential pressure transducer is employed for this purpose, the negative leg of which is connected to the smallest throttling cross-section of the compressor intake. The positive leg detects the pressure in the vicinity of the compressor intake flange; i.e., in a region of wide flow cross-section.
In this case, compressor surges are detected by monitoring the output signal of the differential pressure transducer for a pressure drop below a given minimum differential pressure. In carrying out this method, the differential pressure transducer may be replaced by a differential pressure switch which produces a signal whenever the differential pressure falls below a given presettable value.
However, this method likewise suffers from the drawback that, with too fine an adjustment, pressure surges are indicated even if no surges have occurred, whereas with too coarse an adjustment, surges cannot be detected at all.
Finally, it should be noted that both the flow rate signal and the velocity signal are superimposed on a "noise" signal due to whirls at the pressure tapping points. This leads to a fluctuating measured signal even at steady flow conditions.